Most conductive polymers display a more or less marked rise in conductivity as temperature increases, which shows them to be non-metallic conductors. A few representatives of this substance class display a metallic behaviour at least in a temperature range close to room temperature inasmuch as the conductivity falls as temperature increases. A further method of recognizing metallic behaviour consists of plotting the so-called “reduced activation energy” of conductivity against the temperature at low temperatures (down close to 0° K.). Conductors with a metallic contribution to the conductivity display a positive rise in the curve at a low temperature. Such substances are called “organic metals”.
Such an organic metal has been described by Waling et al. in Eur. Phys. J. E 2, 2000, 207-210. The transition from the state of a non-metallic to an at least partly metallic conductor was effected by a one-stage trituration or dispersion process completed once synthesis of the intrinsically conductive polymer was completed, the process-engineering basis for which is described in EP-A-0 700 573. Conductivity is also increased by the dispersion process, without the chemical composition of the conductive polymer used being substantially changed.
The state of the art contains numerous efforts to clearly increase conductivity. Whereas a conductivity range of around and less than 5 S/cm is normally achieved after synthesis, values of 10s, sometimes also 100s, of S/cm are achieved using different procedures. Conductivity values of 1,000s or 10,000s of S/cm, as achieved by Naarmann and Theophilou in Synthet. Met., 22, 1 (1987) 15 years ago with polyacetylene using a special polymerization process followed by stretching, have not proved achievable to date with other conductive polymer systems. However, the process used by Naarmann et al. has the disadvantage that it is difficult to carry out and difficult to reproduce. Moreover, it leads to a product which is not air- or oxidation-resistant and moreover cannot be further processed.
Apart from the above-mentioned one-stage process of EP-A-0 700 573, the processes of the state of the art are characterized in that selected dopants or selected combinations of dopants are used, often followed by a stretching of the obtained product. Synthetic Metal (Special Issue, Vol. 65, Nos. 2-3, August 1994) and also the contributions by Epstein et al. and Heeger et al. (Handbook of Conductive Polymers, Skotheim, Eisenbanner, Reynolds (Publ.), M. Dekker, N.Y. 1998) give a good overview of these processes.
FIG. 3.2 in Kohlman and Epstein in the above-named handbook gives a very good overview of the achieved conductivity values to date, wherein the higher values around 102 S/cm are generally achieved only after stretching a film or fibre prepared from the intrinsically conductive polymer.
The procedure in the case of polyaniline is e.g. that aniline is polymerized in aqueous hydrochloric acid, whereupon the chloride salt of the protonated polyaniline forms. This is neutralized by means of a strong base, e.g. ammonia, to remove HCl. As a result, the so-called emeraldine base is obtained. This is dissolved with camphorsulphonic acid in the presence of the toxic m-cresol in xylene or chloroform. A film is cast from this solution and then stretched. After stretching, a conductivity of 102s of S/cm is achieved.
This process, called secondary doping, see Mac Diarid and Epstein, Synth. Met. (Special Issue) Vol. 65, Nos. 2-3, August 1994, pp. 103-116, is carried out in numerous variants, i.a. in works by Holland, Monkman et al. J. Phys. Condens. Matter 8 (1996), 2991-3002 or Dufour, Pron et al., Synth. Met. (2003), No. 133-136, pp. 63-68, wherein the acid and the secondary dopant are varied. In further variants of this process, Mattes et al., U.S. Pat. No. 6,123,883, produced fibres which likewise display a conductivity of 102 s of S/cm after stretching.
The state of art discussed above shows that camphorsulphonic acid is regarded as the doping agent of choice.
Likewise it is clear that most researchers try to prepare true solutions of conductive polymers and seek to maximize crystallinity after removal of the solvent.
In view of the numerous publications, it is astonishing that no products with these conductivity properties are commercially available. This is due above all to the fact that the conductivity is not sufficiently reproducible, but also to the fact that toxic solvents or dispersants must be used and that the product must still be stretched.
However, the above-mentioned dispersion process of EP-A-0 700 573 which does not have these disadvantages has likewise not proved to be the best solution to the commercial preparation of end-products with a conductivity of clearly more than 100 S/cm. Added to this is the fact that the end-product is preferably a thermoplastic polymer blend which has a conductive polymer concentration of, generally, only just under 40%. Further processing into products, e.g. layers which either consist predominantly of the conductive polymer or contain any matrix in any concentration (depending on the demands made of the product to be prepared with same), is therefore not possible.